The invention pertains to the art of self-propelled, rigid framed, skid-steered end loaders having dual variable speed hydrostatic transmissions independently controlled for both forward and reverse drive of the loader wheels on opposite sides for propelling and maneuvering the machine.
1. Field of the Invention
A skid-steered loader of the type referred to above is disclosed in U.S. Pat. No. 3,635,365 issued Jan. 18, 1972, to the assignee of the present invention entitled "Tractor Vehicle with Hydrostatic Drive Means", inventor James J. Bauer. Hydrostatically driven loader vehicles of the type described in the aforementioned patent are characterized by the fact that the wheels on one side may be driven in either forward or reverse direction independently of the wheels on the opposite side so that the vehicle is both propelled and maneuvered without actually turning the wheels relative to the body. Rather by varying the speed and/or direction at which the wheels on one side are driven relative to the wheels on the opposite side the machine is "steered" in a skid-steered fashion characterized by quick turns and high maneuverability. The pairs of wheels on each side are driven by separate transmissions. Each transmission is controlled by a separate lever which through a linkage varies the plane of the swash plate of a variable displacement hydrostatic pump having a rotating piston group driven by the engine. The plane of the pump swash plate determines the displacement of the pistons and thus the speed. By varying the angle of the swash plate in either direction from a neutral mode, a change in direction of the transmission units is obtained to drive the wheels in either a forward or reverse manner.
The hydrostatic pumps supply hydraulic pressure to a pair of hydrostatic motors which have output shafts drivingly connected to gear boxes on either side of the machine supplying power to the wheels. Each of the hydrostatic motors, as with the variable displacement hydrostatic pumps, has a rotating piston group and inclinable swash plate. The motor swash plate will be inclined to its full displacement position when the maximum tractive effort of the machine is required, such as during loading operations when speed requirements are low. At times, however, when the machine is to be moved considerable distances, such as between work sites or over a road, higher speeds are needed. In this case the motor swash plates will be inclined at a reduced piston displacement position to increase the speed. Thus, the operator may select either a high or low speed range and within those ranges, vary the speed of the loader by changing the angle of the pump swash plates to obtain a widely variable range of power versus speed ratios depending upon the operational requirements of the machine.
2. Description of the Prior Art
Heretofore it has been the practice to employ a servo mechanism for shifting the hydrostatic motors between the low and high speed ranges. By servo mechanism it is understood that some type of power assisted device is employed for holding the motor swash plates against the force of the pistons attempting to return them to a more reduced displacement position as opposed to a direct mechanical system where the operator must physically hold the angle of the swash plates. In the case of the aforementioned loader the servo mechanism is actuated by twisting a hand grip of one of the steering control levers which actuates a cable connected to a valve. The valve controls an hydraulic cylinder connected to the swash plates of the hydrostatic motors. The operator shifts between either the high or low speed ranges by simply moving the valve spool and the restraining force required for holding the swash plates in the correct position is supplied by the hydraulic cylinder. Also, it has been found that ordinary mechanical linkages do not furnish adequate firm location of the swash plates against the internal forces generated.